1. Field of the Invention
The present invention relates to a conveyor device and film formation apparatus for a flexible substrate.
2. Description of the Related Art
Processes capable of manufacturing at low cost in the mass production of thin film solar batteries (solar cells) have been anticipated in recent years. A method for continuous processing, in which unit operations, such as film deposition, printing, and laser processing, are performed in-line while unwinding a rolled-up flexible substrate and winding it up onto another roll is known as one of measures for reducing production cost. This method is called a roll-to-roll method (hereafter referred to as roll-to-roll).
In particular, performing continuous conveying and continuous film formation using a film formation apparatus that is provided with a roll-to-roll type conveyor device such as that shown in, for example, Japanese Patent Laid-open No. Sho 58-216475 or Japanese Patent Laid-open No. Sho 59-34668, is effective as a means for increasing productivity of a thin film formation process.
Continuous film formation is performed while continuously conveying a flexible film substrate in a film formation apparatus having a roll-to-roll type conveyor device. In order to efficiently form a film to an objective thickness, one method is to lengthen a film formation electric discharge electrode and increase the conveyor speed.
However, when using a generally small size, low cost parallel plate method film formation apparatus for depositing a film to a flexible substrate while conveying the substrate by a conveyor device, wrinkles in the substrate become a cause of irregular film formation, which is a problem. A film formation apparatus in accordance with a parallel plate method is shown in FIGS. 1A and 1B. FIG. 1A is a side face of the entire film formation apparatus, and FIG. 1B is the vicinity, of an electrode 108 and a flexible substrate 101 as seen from below. The electrode 108 is grounded, and a heater is incorporated therein to heat the flexible substrate 101 as needed. Regarding a method of setting the substrate, first the flexible substrate is rolled out from a roll-out roll 105, the flexible substrate passes through gaps 103 formed in substrate conveying portion side faces of a roll-out vacuum chamber 110 and a film deposition vacuum chamber 102, the flexible substrate passes between the electrode 108 and an opposing electrode 109, passes through gaps 112 on the right side of the film formation vacuum chamber, and then, is rolled onto a roll-up roll 104. In order to maintain the substrate in parallel with the electrodes, a constant rotational torque is generated in the roll-up roll 104 and the roll-out roll 105, and a tensile force is applied to the substrate. The substrate is in a state of being suspended between guide rollers 106 and 107 at this time. Further, the flexible substrate stretches and shrinks, and therefore a lengthening force exists in the direction in which the substrate is being conveyed, and a contracting force exists in the width direction, in every portion of the flexible substrate suspended in the air and under application of the tensile force. This causes wrinkle 111 in the substrate. The expansion and shrinkage of the flexible substrate become large when heated by the heater, and wrinkle appears conspicuously. Furthermore, the film is formed with the portion that has been wrinkled exposed to an electric discharge, and this is therefore a cause of uneven film formation. The longer the electrode, namely the longer the portion of the substrate suspended in the air, the higher the frequency of wrinkle becomes.
One method for stopping the flexible substrate from wrinkling is a cylindrical can method. A film formation apparatus provided with a conveyor device in accordance with the cylindrical can method is shown, for example, in Japanese Patent Laid-open No. Sho 58-216475. By applying a tensile force to a flexible substrate, and bringing the substrate into close contact with a curved surface of a cylindrical can, which supports conveyance of the substrate, the wrinkles in the substrate can be suppressed. With a conventional cylindrical can method, a region used as a film formation grounding electrode is a portion of the cylindrical can, and the apparatus becomes larger in size in proportion to the surface area of the electrode. This increase in size is noticeable particularly in a multi-chamber type film formation apparatus in which a plurality of vacuum chambers are connected in a row in order to perform continuous film formation of PIN layers for forming a solar battery.
The increase in size cannot be avoided in a film deposition apparatus provided with a conveyor device of cylindrical can method, but in thinking about making the apparatus smaller, a method of using an improved parallel plate method can be considered. In this improved method, the portion contacting the flexible substrate and supporting the conveyance of the substrate may be made into a curved shape. A conveyor device using a curved surface electrode as a conveyance supporting portion, and a film formation apparatus provided with the conveyor device are shown in FIGS. 2A and 2B, respectively. A curved surface electrode 201 serves as both a conveyance supporting portion and an electric discharge grounding electrode. By applying a tensile force to a flexible substrate 204, the substrate is brought into close contact with the curved surface electrode, and wrinkles in the substrate can be suppressed. This method is remarkably superior to the cylindrical can method with respect to the point of making the apparatus smaller, and makes the apparatus a similar size comparable to that of the parallel plate method. However, a problem is that if the substrate is conveyed while a tensile force is applied thereto, the substrate, being in contact with the curved surface electrode during the conveyance, receives in its back surface abrasions due to rubbing between the back surface of the flexible substrate and the curved surface electrode. Another problem is that the longer the electrode becomes, the larger the friction force grows, which increases a force for winding up the substrate during conveyance and also increases a force working on the substrate to a considerable degree.